This invention relates generally to semiconductors and more particularly to dynamic random access memories (DRAMs).
As is known in the art, DRAMs are made of storage cells having two main components: a storage capacitor; and, a transistor for enabling the transfer of charge to and from the capacitor. In trench type DRAMs, the capacitor is located in a deep trench which is etched into a semiconductor substrate. More particularly, the trench has a conductive material which provides a first electrode (i.e., sometimes referred to a the storage node) for the capacitor. The trench has a dielectric on its walls which provides the dielectric for the capacitor. A doped region in the substrate provides the second electrode for the capacitor. The first electrode is coupled to one of the source/drain regions of the cell's transistor through a coupling region formed in the semiconductor between an upper portion of the conductive region and a buried strap formed in the substrate.
Conventional DRAM arrays are organized so that multiple storage cells are positioned as close as possible to one another. In order to operate properly, it is essential that the transistor of one cell be electrically isolated from the transistor of an adjacent cell. These transistors are formed in active areas of the semiconductor substrate. These active areas are defined by a masking and etching process. One such process is sometimes referred to as shallow trench isolation (STI). More particularly, as described in U.S. Pat. No. 5,717,628 entitled Nitride Cap Formation in DRAM Capacitors, issued Feb. 10, 1998, after forming the trench capacitor, a vertical area is etched into the electrically active silicon substrate and filled with oxide. This etching of the vertical area is typically performed using a mask which must be properly aligned with the trench capacitor. More particularly, as the mask becomes positioned closer to the transistor region, the resistance of the coupling region increases; whereas if the mask becomes positioned further from the transistor region of the cell, the likelihood that the active areas of two adjacent cells becoming electrically connected to one capacitor increases.